First of all it is shown the BER before FEC (Forward Error Correction).
In a digital reception system for cable signals, after the QAM decoder an error correction method called Reed-Solomon is applied (see
figure 27). Obviously, the error rate after the corrector is lower to the error rate at the QAM decoder output. This is the reason because this
screen provides the BER measurement before FEC (Forward Error Correction) and the number of non correctable errors (W.P., Wrong
Packets) received after Reed-Solomon in the measuring time.
Figure 27.- Digital reception system via cable.
The BER measurement is provided in scientific notation (i.e. 1.0 E-5 means 1.0x10
-5
that is to say one wrong bit of every 100,000) and
through an analogue bar (as its length is smaller the signal quality will be better). The analogue representation is done on a logarithmic scale
(not linear).
With the aim to have a reference about the signal quality, it is considered that a system has a good quality when it decodes less than one
non-correctable error for every transmission hour. This border is known as QEF (Quasi-Error-Free) and it corresponds approximately to a
BER before FEC of 2.0E-4 BER (2.0x10
-4
, that is to say two incorrect bits of every 10,000). This value is marked on the measurement bar of
the BER and therefore, BER for acceptable signals must be at the left side of this mark.
Below the BER analogue bar it is shown the tuned frequency (or channel) and the frequency deviation in kHz between the tuned
frequency and the one which optimizes the BER (i.e. 800.00 MHz + 1.2 kHz).
In the next line it is shown the number of non corrigible packets received 'wrong packets' (up to a maximum of 126) in the time detailed
on its right. A packet is considered wrong when one, at least, non corrigible bit is detected. To reset this measurement just modify the
measurement conditions: for example change the tuned frequency.
Finally it is shown a status line which displays information about the detected signal. The possible messages that can appear and its
meaning are shown in the following list. The messages are exposed from less to more fulfilment of the MPEG-2 standard:
No signal received
Any signal has been detected.
Signal received
A signal is detected but it can not be decoded.
Carrier recovered
A digital carrier has been detected but it can not be decoded.
MPEG-2
Correct detection of a MPEG-2 signal. The BER is showed.
In case of detecting a DVB signal, message MPEG-2 DVB-C will appear and the DVB Channel Identifier function will be autoatically
activated. See section 5.4.1 DVB Channel Identifier.
IMPORTANT REMARK
DVB-C channels tuning may require an adjusting process. It is recommended to follow next procedure:
1.- From the spectrum analyser mode, tune the channel at its central frequency.
2.- Switch to TV mode, BER measuring mode.
3.- If in the lower line of the screen does not appear MPEG-2 message (and consequently BER is unacceptable), by pressing keys
[6] deviate the tuning frequency until MPEG-2 message appears. Finally tune channel again to minimize the frequency
deviation which optimizes the BER and therefore minimize the BER.
If it is not possible to detect any MPEG-2 channel, make sure that digital signal parameters are correctly defined and if signal level is too
low, check that the 30 dB attenuator is not activated (Attenuator 0 dB).
TUNER
QAM
DEMOD.
A/D
REED-
SOLOMON
BER before FEC
Wrong packets
142
TV & SAT Level Meter MTD 120
User Manual
